Coronary angiography is an important procedure in the diagnosis of medical problems associated with the coronary arteries that supply blood to the heart. During this procedure, the coronary arteries are imaged to enable a medical practitioner to observe any blood circulation problems that may affect the heart.
A radio-opaque contrast substance injected into the coronary arteries during the angiography procedure causes the arteries to appear as bright lines against a relatively darker background. Where a restriction (stenosis) has occurred in a coronary artery, the artery will appear to be pinched, i.e., it will have a smaller cross-sectional thickness at the location of the restriction. Since the heart is three-dimensional, a stenosis in a coronary artery may not be evident from certain viewing angles. It is typically necessary to produce at least five angiography sequences, each at a different projection angle relative to the heart, to ensure that all portions of the coronary arterial system are visually presented for accurate medical diagnosis.
During each of the angiography sequences in which the radio-opaque contrast substance is injected into one of the coronary arteries, from 150 to 250 consecutive frames are recorded with a cine camera and/or a video camera, or in a digital format. Each sequence records from five to 15 cardiac cycles or heart beats. During each beat, the heart ventricles fill with blood during diastole, reaching their maximum volume at end diastole. The heart muscle contracts during systole, and the ventricles reach their minimum volume at end systole. Most of the filling of the coronary arteries with blood takes place during diastole, because the coronary arteries pass through the heart muscle, and the pressure exerted by the contracting muscle during systole tends to impede blood flow through the arteries. During the imaging sequence, the injected radio-opaque contrast substance can be seen to fill the coronary artery and then to gradually clear from the artery as fresh blood, which does not contain the radio-opaque contrast substance, enters the arteries.
When making a diagnostic analysis of the images in accord with clinical procedures, a physician will either estimate the severity of coronary artery stenosis based on a visual examination of the images, or perform a quantitative analysis of the artery lumen dimension at the point of maximum stenosis and in the adjacent normal anew segment(s) disposed above and/or below the stenosis. Commercially available software programs designed to partially automate the measurements of these artery dimensions may be used. For either analysis, a physician will typically view all of the frames in a sequence and then repetitively view some of the frames that appear to include the best images of the coronary anew at end diastole. The frames of greatest interest are those that show the stenotic segment and the adjacent normal segment(s) clearly, free of overlying branch vessels or other structures, with minimal foreshortening, without blurring due to motion, and when the anew is filled with the radio-opaque contrast substance. This condition occurs most frequently at end diastole, because at that point in the cardiac cycle, the increased volume of the heart separates the branches from each other, there are few motion artifacts as the heart pauses to change direction from moving outwardly to moving inwardly, and the artery is not compressed by muscle contraction. Occasionally, the stenotic artery segment may be most clearly visualized in the frame following end systole, which is another time at which there are few motion artifacts as the heart pauses to change direction from moving inwardly to moving outwardly. The physician will thus likely select a preferred image showing the coronary anew at end diastole or possibly, at end systole, for further analysis.
Conventionally, the physician only performs diagnostic analysis on selected images from the one sequence that best shows the stenotic artery segment. Alternatively, the same steps may be again manually implemented by the physician to select one or more preferred frames in other angiography sequences.
In some hospitals, the quantitative analysis may be performed by medical staff who are not physicians, such as radiology technicians. Also, the analysis may not be performed until some time following the coronary angiography procedure, for example, when the physician has completed the examination and is dictating a report for the patient's files.
The time required for the physician or technician to review the frames in an angiography sequence to select those images for further analysis varies, depending on the quality of the images, the skill of the medical practitioner, and the criteria applied in making the selection. In some cases, only a few minutes will be required to manually select specific images from an angiography sequence. However, even the relatively short time required to manually make the selection can be significant, particularly if the patient is waiting to undergo further angiography sequences, pending a decision that the severity of coronary stenosis is sufficient to warrant a change in treatment, or that the treatment applied in a cardiac catheterization facility has adequately reduced the stenosis.
Another problem that arises in connection with the current practice of manually selecting the preferred frames in a sequence of angiogram images for analysis relates to the problem of integrating the angiogram image data with other information, such as the patient's history, and of maintaining and searching the image data at a later time. Image data produced during an angiography procedure are sometimes digitized to facilitate automated analysis and then are indexed and stored. However, the files in which digitized image data for a complete sequence of angiogram images are stored can be quite large. Even when compression techniques such as those developed by the Joint Photographic Expert Group (IPEG) or by the Motion Picture Expert Group (MPEG) are used to reduce the file sizes, the amount of digital image data produced by digitizing a complete angiography sequence can quickly fill available digital storage resources. Ideally, only the selected frames from each sequence should be stored, along with indexing information that indicates the parameters associated with the selected frames. A patient identifier, the date of the angiography procedure, the sequence number(s) from which the selected frames are derived, the frame number of the selected frames, and the digital image data for the selected frames should provide a sufficient record to support any subsequent diagnosis made from the selected frames. The conventional approach in which angiography images are manually reviewed and selected for further analysis does not readily permit such a record to be produced and maintained.
Based on the preceding remarks, it will be evident that at least an initial selection of the angiography images that will be used for further analysis and diagnosis should be automated. By automating the selection process, the time required to start the diagnosis can be greatly reduced. In addition, the automated process should improve the quality of the selection process by minimizing subjective criteria that may incorrectly influence the choice of a frame in an angiography sequence. If the selection process is thus automated, indexing of the selected frames for storage with a patient's medical history is easily implemented.